High-temperature thermal storage device with induction heating and molten metal, and thermal storage-composite system

ABSTRACT

A high-temperature thermal storage device has a crucible, an induction heating device, a tubular duct in which a liquid and/or gaseous heat carrier flows, and an electrically conductive storage medium filled into the crucible that fills the crucible at least partially. The induction heating device, when electric energy is applied, heats the storage medium. The tubular duct passes at least partially through the storage medium. The storage medium is a molten metal and the thermal storage device has an operating temperature exceeding 700° C. The thermal storage device readily provides steam, be it saturated steam or superheated steam, fulfilling all parameters required for the operation of a conventional thermal power station.

BACKGROUND OF THE INVENTION

The invention relates to a high-temperature thermal storage device with induction heating, a thermal storage composite system and a process for operating the high-temperature thermal storage device and the thermal storage composite system.

In the context of the invention, the term “storage device” is used synonymously with the term “thermal storage device”.

When storing thermal energy in the form of high temperature heat, exergy is lost during charging and discharging of the thermal energy storage device.

This exergy loss arises because the temperature level, at which the storage device was charged, is no longer attained when discharging the storage device. The magnitude of this effect depends on the storage technology selected, the (heat carrier) media used, and the design of the heat exchangers involved.

In thermal storage devices tested and used to date for solar thermal power stations, charging of the storage material is performed by heat transfer from one heat carrier medium (e.g. steam or air or thermal oil) to the storage medium (e.g. liquefied salt or concrete). The temperature of the storage material, due to the losses caused by the heat transfer, is in this case lower than the temperature of the heat carrier medium emitting the heat. During discharging of the storage device, this process is performed in the opposite direction. In this case, the temperature decreases once again due to the heat transfer. As a result thereof, exergy losses arise both during charging as well as during discharging of the storage device.

A water/steam circulatory process has been used for decades in thermal power stations. The efficiency of the circulatory process depends in this context, inter alia, on the steam parameters (pressure and temperature) of the steam at the turbine inlet. This is to say, if the steam parameters at the turbine inlet are no longer attained due to the exergy losses in a thermal storage device, the output and efficiency of the steam turbine will decrease and, consequently, also that of the power station.

Thermal power stations are nowadays not only fossil fuel-fired, but the steam is frequently generated by means of solar radiation. In this context, inter alia, parabolic trough collectors or other concentrating solar collectors are used. In order to attain the design parameters of the steam turbine process in a solar thermal power station even at low solar radiation, supplementary combustion means are often installed as separate steam generators or as booster combustion means with fossil fuels.

DE 21 00 485 A1 describes an evaporator for a fast breeder nuclear reactor which is operated with liquid metal. In this case, saturated steam is generated for a steam turbine process by means of liquid sodium. The melting temperature of sodium is at 98° C. The boiling temperature of sodium is approximately 880° C. The steam temperature in a fast breeder is between 345° C. and 545° C. The steam temperature ahead of the steam turbine is 487° C., the steam pressure is 177 bar (data from Superphenix I, France). The evaporator is designed as a nested tube heat exchanger, to which liquid metal is applied on the primary side, while water evaporates and superheats on the secondary side as a result of the heat transfer. Heat for liquefying the sodium is supplied inside the nuclear reactor.

DE 28 51 97 A1 describes the integration of intermediate superheating in a steam converter, which is operated with liquid metal.

It is the object of the invention to provide a high-temperature thermal storage device which allows the temperature level of the steam, generated during discharging of the storage device, to be raised again to the temperature level required for the operation of the power station, or even beyond that temperature level, without using any fossil energy carriers.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by a high-temperature thermal storage device, including a crucible, an induction heating device, and a tubular duct for a liquid and/or gaseous heat carrier (e.g. steam), wherein the crucible is filled, at least in part, with an electrically conductive storage medium, wherein the induction heating device, when under current, heats the storage medium, and wherein the tubular duct passes through the storage medium, at least in part.

This high-temperature thermal storage device according to the invention has many parallels to induction furnaces, as they are used, for example, in aluminium foundries. For this reason, many components, such as, for example, the crucible and the induction heating device, are commercially available and decades of operating experience exist which can be utilized for the high-temperature thermal storage device according to the invention. In contrast to an induction furnace, the crucible need not be pivotal, as the storage medium, normally in a liquid state, inside the crucible need not be changed. In addition, in contrast to a conventional induction furnace, a tubular duct is provided which passes through the storage medium, at least in part. A particularity of the high-temperature thermal storage device resides in that heat during charging is supplied in a manner different from the heat dissipation during discharging.

During normal operation, i.e. when the storage medium is present in the liquid state, the storage medium is heated by means of the induction heating device. This process is performed also when using an induction furnace during melting and heating up of the metal present in the crucible.

As the exergy content of the electric energy, used for heating up the storage medium, is virtually 1 or equal to 1, there are no thermodynamic restrictions with regard to the operating temperature of the high-temperature thermal storage device according to the invention. Of course, it must be ensured that the temperature of the storage medium is not unnecessarily high and that the components of the high-temperature thermal storage device, such as, in particular, the crucible and the tubular duct, do not suffer any damage due to overheating.

Discharging of the high-temperature thermal storage device according to the invention is performed in that a liquid or gaseous heat carrier passes through the tubular duct. As a rule, this will be steam, which is used in the water/steam cycle of a thermal power station.

As the storage medium with the induction heating device can be heated to a temperature of, for example, 770°, it is possible, without any problem, to heat the heat carrier (steam), flowing through the tubular duct, from an inlet temperature of, for example, 450° C. to an outlet temperature of 550° C. or 600° C. It is, therefore, possible to generate superheated steam, which is then supplied to a steam turbine, either directly or after mixing with saturated steam having a lower temperature.

In view of the fact that charging of the high-temperature storage device according to the invention is performed by using electric energy and the storage medium tolerates very high temperatures, the high-temperature thermal storage device according to the invention can operate at a very high temperature. It may be considerably higher than the temperature of the steam at the turbine inlet. This allows bringing the steam to the temperature required at the turbine inlet when discharging the high-temperature thermal storage device, thereby ensuring a turbine operation with optimized output and optimum efficiency.

It has proved advantageous, with regard to safety and cost, to use a molten metal as the storage medium.

The term “molten metal” is used because in normal operation the storage medium is in liquid form. Pure metals or metal alloys, the melting points of which are approximately in the range of 500° C. or above 600° C., are suitable as molten metals.

It is possible, for example, to use a commercially available aluminium alloy to serve as molten metal or storage medium, respectively. Such aluminium alloys have a melting temperature exceeding 600° C.

Molten aluminium generated in foundries in conventional induction furnaces has temperatures ranging between 680° C. and 780° C. If copper or other metals or metal alloys are used, even higher temperatures can be attained. Depending on the design, the construction material for the tubular duct, and process control, the steam may in this case be heated to approximately 720° C. As molten metals and steam have very good thermal conductivities, and because the temperature difference between the molten metal and the steam passing through the tubular duct is relatively high, the portion of the tubular duct that is present in the molten metal suffices in many cases to serve as heat exchanger. This applies, in particular, when this portion of the tubular duct is not configured straight, but in meandering fashion.

Should the heat-transferring surface of the portion of the tubular duct present in the molten metal not suffice, a heat exchanger, a nested tube heat exchanger, for example, can be integrated in the tubular duct in order to ensure the heat transfer from the molten metal to the steam flowing inside the tubular duct.

When the molten metal is liquid, the heat transfer between the storage medium and the tubular duct, in which the heat carrier flows, is particularly favorable and the heat carrier flowing in the tubular duct can, therefore, be heated to virtually the same temperature as the molten metal.

Any build-up of a solid layer of the molten metal on the tubular duct presents no problem, as these molten metals have very good thermal conductivity.

It is a further important advantage of the high-temperature thermal storage device according to the invention that the storage device can be readily shut down. In this case, the storage medium solidifies as soon as it has dropped below the melting temperature. This is a non-critical event because neither the crucible nor the tubular duct will suffer any damage as a result thereof. When the high-temperature thermal storage device according to the invention is to be put back into operation, it is quite possible, with the aid of the induction heating device, to re-heat the solidified molten metal and to transform it to the liquid state. This is an important advantage of the high-temperature thermal storage device according to the invention in contrast to the high-temperature thermal storage devices known from the prior art which operate with molten salt. Once this molten salt has solidified, it is virtually impossible, due to the poor conductivity of the salt, to re-liquefy this molten salt.

The high-temperature thermal storage device according to the invention is thus also very simple with regard to the manner of operation, is robust, and problem-free. This is particularly important because such high-temperature thermal storage device should be operated for several decades around the clock.

The high-temperature thermal storage device according to the invention has still further advantages with regard to the operating manner of a thermal power station, into which a high-temperature thermal storage device according to the invention has been integrated.

It is possible, for example, to utilize the output of the induction heating device as control output for stabilizing the composite grid. If, for example, excess electric energy exists, due to the fluctuating generation of renewable energies, the output of the induction heating device may be increased at very short notice. “Excess” power is then used in order to charge the high-temperature thermal storage device. The purchasing costs for this “excess” power are very low.

In addition, it is, of course, also possible that in the event of a short-term shortage of electric energy, the induction heating device can likewise be switched off at very short notice or the output reduced so that the entire power generated by the power station can be fed into the grid. Considerably higher prices can be attained for such electricity so that, from an economic point of view, the use of the high-temperature thermal storage device is very attractive for providing demand energy.

When the high-temperature thermal storage device according to the invention forms part of a solar thermal power station, solar radiation is captured during the day by means of a collector array; said solar radiation can be used for heating the high-temperature thermal storage device.

During the day so much solar energy can be coupled into the water/steam cycle of the power station by means of the collector array that the power station does not only generate electric energy, which is fed into the public grid, but, in addition, electric energy can further be used for heating the high-temperature thermal storage device according to the invention. At the end of the day, when the output of the collector array decreases naturally and finally reaches zero, the high-temperature thermal storage device is fully charged so that steam can even be generated at night, the parameters (pressure and temperature) of which are as high as during the day, such that the turbine of the power station can be operated at optimized capacity and optimum efficiency. Therefore, a solar thermal power station which is capable of handling base loads is provided. This base-load capability is made possible without employing any fossil fuels.

Using this high-temperature thermal storage device according to the invention enables a solar thermal power station to perform, in a simple manner, under black-out conditions, i.e. to actively cooperate in re-establishing the voltage of the high voltage grid in the event of a grid failure, along with other power-generating units.

It is also possible to charge the induction heating device and the thermal storage device simultaneously by using the induction heating device and to discharge it simultaneously via the heat carrier flowing inside the tubular duct. This ensures very high flexibility in the operation of the power station. Generally, this makes no sense energetically. However, this operating mode may result in economic advantages for the power station operator. In addition, the grid stability of the composite grid may be increased.

It goes without saying that the high-temperature thermal storage device according to the invention is provided with thermal insulation, which, as a general rule, is applied as an exterior insulation. Heat-resistant mineral wool or ceramic wool, customary in power station construction, may be used as insulation material.

In order to allow operation of a solar thermal power station around the clock with optimum efficiency, minimal storage losses, and low investment and operating costs, it is recommended to employ a thermal storage composite system, comprising a first thermal storage device and a second thermal storage device, wherein the first thermal storage device is discharged by a liquid or gaseous heat carrier and wherein subsequently at least a partial flow of the heat carrier flows through the second thermal storage device and is thereby heated up further.

Ideally, the second thermal storage device is then utilized to further superheat the steam upstream of the steam turbine. Approximately 5-25% of the overall stored heat is thus stored in the second storage device, while the remaining 95-75% are being stored in the first storage device at a lower temperature level.

The second thermal storage device is a high-temperature thermal storage device. The coupling of two thermal storage devices having different operating temperatures allows minimizing the exergy losses because the heat is available at sufficiently high temperatures, and, on the other hand, because the temperatures of the stored heat are not unnecessarily high. This further minimizes investment and operating costs.

It goes without saying that, with the thermal storage composite system according to the invention, it is not necessary for the entire mass flow of the heat carrier to be passed through the second thermal storage device. It is, of course, also possible to guide only a partial flow of the heat carrier via the second high-temperature thermal storage device. The heat carrier (normally steam) is superheated there. The superheated partial flow is subsequently mixed with the residual flow of the heat carrier in an injection station, and the desired temperature of the heat carrier adjusted as a result thereof. The residual flow of the heat carrier is heated only by using the heat stored in the first thermal storage device.

It is, of course, also possible to operate a plurality of molten metal thermal storage devices in parallel or arranged in series. The use of different metals or metal alloys, arranged in series, opens up further possibilities for operational optimization.

This possibility of guiding a partial flow of the heat carrier via the second thermal storage device further allows, in the event of an operational failure of the second heat exchanger, to only employ the first heat exchanger, thereby maintaining the operation of the power station, albeit with reduced efficiency and decreased output.

The thermal storage composite system according to the invention opens up a multitude of possibilities for optimizing the operation of a solar power station.

As electric energy for charging the molten metal, the solar-generated energy can be used either during day operation and/or also during the discharging operation. In this manner, heating up the molten metal may, for example, be dispensed with during daytime operation with high power requirement; heating may then be done during discharging to serve as power supply for own use.

If more solar power is to be generated at night, the molten metal is charged during day operation only with heat generated by solar power, or heating up the molten metal is spread over daytime and night-time operation.

An operating mode wherein the high-temperature thermal storage device is charged with electric energy and simultaneously discharged via the steam in the tubular duct may be useful during the day, for example, when clouds are passing through, when it is nonetheless desired to have the high-temperature thermal storage device also at an appropriately high charge state at night. This means further flexibility, i.e. during the day the solar power station can even be operated when clouds pass through and it is not necessary to abandon the planned power production during the night or to limit the latter appreciably.

Making use of this complementary process allows a solar power station to offer supportive capacities for the power grid operator. For example, during charging of the molten metal, a decision may be made at short notice to switch off or to operate, as needed, the induction heating device in favor of secondary load-balancing or minute reserve.

Excess power from other renewable sources, such as wind power, may likewise be used for operating the induction heating device according to the invention.

As during discharging the amount of thermal energy for further heating of the steam for attaining the set steam parameters is usually very low in comparison to the amounts of thermal energy required during preheating and evaporation (as a rule, approximately 5% to 25% max), and molten metals have a very high specific thermal storage capacity, this complementary unit has relatively small dimensions with respect to volume in comparison to the first ‘main’ thermal storage device. This keeps investments costs relatively low as well.

Due to the large temperature gradients between the steam to be heated up further and the molten metal, there is large operational scope with regard to the lowest possible metal temperature. If, for example, aluminium is employed as the metal, it is even possible, due to the very good thermal conductivity, to also use the heat being released during the phase change from the liquid to the solid state at a stable temperature of about 660° C.

Re-melting of the solidified aluminium via the induction furnace poses no problem either. This is a great advantage, e.g. compared with molten salt, which has a relatively poor heat transfer and which, after large scale solidification, can no longer be melted at reasonable expense.

Because of the low amount of thermal energy required for increasing the temperature during superheating, it is sufficient to guide a partial steam flow through the molten metal bath and heat it up as much as possible. This hot partial flow is subsequently mixed with the colder partial flow, such that the desired steam temperature upstream of the steam turbine then is adjusted to e.g. 540° C. Mixing of the steam flows is normally performed by injecting the hot steam via a steam mixing station, normally employed in power station technology.

In the high-temperature thermal storage device according to the invention and the thermal storage composite system according to the invention, generally commercially available components are used, which are appropriately adapted to the process described herein. In this case, in contrast to the aforementioned publications, energy supply (electric) and melting and liquefying of the metal are performed in the same component as superheating of the steam. For economic reasons, the system proposed herein is used essentially only for further superheating the steam. For storing the large amount of thermal energy, as required for preheating and subsequently evaporating the water (altogether about 75-95% of the amount of energy), a more cost-effective system with a simpler storage medium is employed. Preferably, this is a high-temperature thermal storage device, such as described, e.g. in the PCT publication WO 2012/017041 A2 of 9 Feb. 2012. Since, in principle, all thermal storage devices incur a loss of temperature between charging and discharging, this complementary system can also be employed for all other storage systems (e.g. salt storage devices), thereby replacing the gas or oil tanks used nowadays for compensating for exergy loss.

Further advantages and advantageous embodiments of the invention can be taken from the attached drawings and the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a high-temperature thermal storage device according to the invention.

FIG. 2 is a schematic representation of a thermal storage composite system according to the invention.

FIG. 3 illustrates charging of a thermal storage composite system according to the invention.

FIG. 4 illustrates discharging of a thermal storage composite system of the thermal storage system according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a working example of a high-temperature thermal storage device 1 according to the invention in a highly simplified and schematic section view. The essential components of the high-temperature thermal storage device 1 according to the invention are a crucible 3, an induction heating device 5, a tubular duct 7 as well as molten metal 9 inside the crucible 3. The crucible 3 is a receptacle which is heat-resistant, even if the molten metal 9 is liquid, and, for example, has temperatures exceeding 700° C. In order to minimize heat loss, the crucible 3 includes a lid (without reference number) at its upper end.

The sidewalls of the crucible 3 are surrounded by the induction heating device 5. Such induction heating devices 5 are often used in induction furnaces and consist substantially of one or a plurality of coils 11 as well as sheet metal jacketing 13.

Since such crucibles 3 and induction heating devices 5 are known from induction furnaces for foundries, the knowledge of their construction and function is part of the know-how of a skilled person. For this reason, a detailed description of the technology in connection with the claimed invention is dispensed with.

A molten metal 9, serving as storage medium, is contained inside the crucible 3. An aluminium alloy, a copper alloy or another molten metal may be used as storage medium. It is, of course, recommended in this context to use an alloy which is cost-effective and easy to handle with regard to its functional properties. Long-term stability of the alloy should likewise be taken into account, since the high-temperature thermal storage device according to the invention will be operated for many years and the storage medium should preferably not be replaced.

A tubular duct 7 is passed into the crucible. A portion of the tubular duct 7 is immersed in the molten metal 9. This portion of the tubular duct 7 therefore serves as a heat exchanger 10.

A heat carrier, for example steam, flows in the tubular duct 7 and absorbs heat from the molten metal 9 when passing through the heat exchanger 10, thus reaching a higher temperature.

Bearing in mind, for example, that steam enters the tubular duct 7 at an inlet temperature T_(in) of 450° C. and flows through the heat exchanger 10 and, therefore, through the molten metal 9, which, for example, has a temperature 770° C., the steam in the heat exchanger 10 is then heated and exits again as superheated steam from the high-temperature thermal storage device 1 at an outlet temperature T_(out) of, for example, 550 to 580° C. It is understood that higher outlet temperatures T_(out) can be attained as well, if this is desired.

It is, however, important that the parameters of the heat carrier when exiting from the high-temperature thermal storage device 1 according to the invention correspond to the desired steam parameters at the turbine inlet or are even above the permissible temperatures at the turbine inlet. In this way, the desired steam parameters at the turbine inlet can be set by mixing with saturated steam or steam at lower temperature or by way of an injection station, usually employed in power station construction.

FIG. 2 shows schematically a working example of a thermal storage composite system according to the invention.

The composite system includes a first thermal storage device 15 and a high-temperature thermal storage device 1 according to the invention, operating as second thermal storage device.

The first high-temperature thermal storage device 15 may be a high-temperature thermal storage device known from the prior art filled with a bed of stones or concrete serving as storage medium. Charging of this first high-temperature thermal storage device 15 is not shown in FIG. 2. Charging is done by way of a heat carrier, such as, for example, air, thermal oil or even steam from the water/steam cycle of a thermal power station (not shown). It is, however, important in the context of the invention that, when e.g. steam having a temperature of 550° C. is used for charging the first high-temperature thermal storage device 15, the heat carrier (steam) during discharging of the first high-temperature thermal storage device 15 can only be heated to a temperature of about 450° C. The difference in temperature of e.g. 550° C.−450° C.=100° C. is attributable to the terminal temperature differences during heat transfer when charging and discharging the first high-temperature thermal storage device 15.

The outlet temperature of the heat carrier is denoted by T_(out) in FIG. 2. In addition, it is assumed that water or steam is used as heat carrier. At an outlet temperature of T_(out,15) of 450° C. and a correspondingly high pressure, the heat carrier is provided, for example, as slightly superheated steam.

A tubular duct 7 of the high-temperature thermal storage device 1 according to the invention branches off a main steam duct 17. A first control device 19 is installed in the tubular duct 7 by means of which the mass flow of the heat carrier can be guided through the tubular duct 7.

In the main steam duct 17 a second control device 21 is provided downstream of the branch of the tubular duct 7 by means of which the mass flow of the heat carrier can be guided through the main steam duct 17.

When, as already mentioned, slightly superheated steam exits from the first high-temperature thermal storage device 15 at a temperature T_(out,15) of e.g. 450° C. and a partial flow is guided through the tubular duct 7, this partial flow is heated in the second high-temperature thermal storage device 1 to a temperature of T_(out,1) of e.g. 630° C. When the temperature T_(out,1) is higher than the admissible or desired steam temperature at the turbine inlet, the steam superheated in the first high-temperature thermal storage device 1 is mixed in a mixing station 23 into the residual flow of the slightly superheated steam flowing in the main steam duct 17 such that, downstream of the mixing station 23, live steam is formed with the desired parameters (pressure and temperature) of e.g. 580° C. The injection of water via a normal injecting station may possibly be necessary for the preparation of live steam.

This live steam may then be supplied directly to a turbine (not shown) and be used for power generation. After the steam has been depressurised in the turbine, not shown, it is condensed in a condenser, likewise not shown, and flows in a condensation line 25 into the first high-temperature thermal storage device 15 at a lower temperature.

A third control device 27 is provided for the condensate in the condensation line 25. It is thus possible, by appropriate triggering of the control devices 19, 21 and 27, to control the mass flow of the condensate or the steam in the ducts 25, 17 and 7 in accordance with the requirements of the power station operation.

During the charging process, the control devices 19, 21 and 27 are closed and no steam flows through the thermal storage device 1. With the aid of the induction heating device 5, the storage medium contained in the crucible 3 is melted and adjusted to the desired temperature (e.g. 700° C.). This process may be so regulated that melting or maintenance of the desired temperature is performed at a time when excess power is available in the power grid and switching-off of the generating plant would be required.

FIG. 2 describes the discharging operation both of the first high-temperature thermal storage device 15 as well as of the second high-temperature thermal storage device 1.

When, for example, sufficient solar radiation is available during the day, the first high-temperature thermal storage device 15 is charged with sensible heat, for example in the form of hot air or steam. Such sensible heat is made available by the solar collectors of the power station.

The second high-temperature thermal storage device 1 is heated up at the same time or with time delay by means of the induction heating device 5. The electric energy required for this purpose can be made available directly by the generator of the solar thermal power station. It is also possible to heat up the induction heating device 5 with electric energy which is available at reasonable prices on the spot market. This allows the purchase of, for example, reasonably priced excess power from wind energy generation and the intermediate storage of this energy in the high-temperature thermal storage device 1 according to the invention.

FIGS. 3 and 4 show a further embodiment of a thermal storage composite system in a more detailed manner. In this case, the first high-temperature thermal storage device 15 is based on the system described in WO 2012/017041 A2. Reference is therefore being had expressly to WO 2012/017041 A2 and attention is drawn to the details and structural particulars additionally as well as the manner of operation described therein.

In FIGS. 3 and 4, the second high-temperature thermal storage device 1 as well as the periphery with the main steam duct 17, the tubular duct 7, and the condensation line 25 are identical to the embodiment according to FIG. 2, such that, in this regard, reference is being had to the description of FIG. 2.

The first high-temperature thermal storage device 15 includes the actual thermal storage device 29, which is filled with a storage medium, such as crushed stone or gravel. The thermal storage device 29 is charged by heated air. This air is taken in from the environment by a blower at ambient conditions and guided via an air/air heat exchanger 31. The air taken in from the environment is preheated in the air/air heat exchanger 31, namely with the air flowing out of the thermal storage device 29. This results in heat recovery which increases the efficiency of the first high-temperature thermal storage device 15.

The preheated air subsequently passes through a charging heat exchanger 33. This charging heat exchanger 33 is connected to a solar collector array via tubular ducts. The charging heat exchanger 33 is consequently supplied with heated thermal oil or steam from the solar collector array. The thermal oil is fed to the preheated air in counter-current, thereby releasing heat to the preheated air. The thermal oil is subsequently returned to the solar collector array and again heated and evaporated there.

At the outlet of the charging heat exchanger 33, the air has a temperature of, for example, 530° C. This very hot air then flows into the heat storage device 29 and heats the storage medium provided therein to a temperature that at least partially exceeds 500° C. When exiting the thermal storage device 29, the air still has a temperature which is considerably higher than the ambient temperature, such that it is to release heat able in the air/air heat exchanger 31 to the taken-in ambient air, thereby preheating the latter in the manner already described above.

With regard to the charging process, it can thus be stated that charging of the thermal storage device 29 can be performed by way of solar-generated thermal oil or steam and that, in this context, temperatures, at least locally, exceeding 500° C. can be attained in the thermal storage device 29. The discharging heat exchanger 35, likewise shown, is deactivated during the charging process, i.e. air does not flow therethrough.

The flow paths of the air are indicated by arrows (without reference numbers). The flow paths result further from the positions of the various valves (without reference numbers) in the air ducts of the first high-temperature thermal storage device 15 according to the invention.

FIG. 4 shows the discharging process. As can be seen, in comparison with FIG. 3, the air is guided differently from the charging process.

During discharging, ambient air is also sucked in via a blower 30, is preheated in the air/air heat exchanger 31, and guided to the thermal storage device 29. The air is heated up in the thermal storage device 29 and exits from the thermal storage device 29, for example, at a temperature a little below 500° C. This hot air is now fed to the discharging heat exchanger 35 and releases heat in the discharging heat exchanger 35 to the condensate resulting from the power station. The condensate is evaporated thereby (and partially superheated). As a result thereof, saturated steam, for example, exits from the main steam duct 17 at a temperature of 450° C. This saturated steam is now passed entirely or partially through the second high-temperature thermal storage device 1 such that upstream of the mixing station 23 superheated steam is available having the desired steam parameters. The discharging process has already been described with regard to FIG. 2.

During discharging of the high-temperature thermal storage composite system according to the invention, the control devices 21 and 27 are open. Feed water flows into the discharging heat exchanger 35 of the first thermal storage device 15 via the control device 27 and is evaporated. A portion of the generated steam is fed via the control device 19 to the high-temperature thermal storage device 1 according to the invention and is passed through the molten metal via the tubular duct 7 and the heat exchanger 10. The steam is heated further in the course thereof. The molten metal bath cools off at the same time.

If the temperature of the molten metal bath is sufficiently high for superheating to the required degree the steam passing therethrough, it is not necessary to switch on the induction heating device 5 so that no electricity is consumed during discharging in the high-temperature thermal storage device 1.

It is evident from the descriptions of FIGS. 2 to 4, that the thermal storage composite system according to the invention offers very favorable operating properties, because most of the heat is stored in the solids thermal storage device 29 that is constructed in a relatively simple manner and has temperatures that are not quite sufficient for operating a steam turbine with optimum efficiency.

This “weakness” of the first high-temperature thermal storage device 15 is compensated for by the second high-temperature thermal storage device 1 according to the invention. As the second high-temperature thermal storage device 1 according to the invention can have an operating temperature clearly exceeding 700° C., it is no problem to reach the required temperatures of the live steam at the turbine inlet. In connection with the claimed invention it is important that charging of the second high-temperature thermal storage device 1 can be performed with electric energy, which is generated in the power station on site. Alternatively, it is also possible to purchase reasonably priced electric energy, for example, on the spot market, to place the latter into intermediate storage in the high-temperature thermal storage device 1 in the form of sensible heat and, for example, to generate electric energy again with this intermediately stored thermal energy during the night.

The discharging process can run until the storage device 1 is either discharged, or until no more steam can be produced at an adequate temperature at the selected operating pressure, or the melting temperature has dropped so low that superheating of the steam is no longer adequate. If the temperature of the molten metal is insufficient for attaining sufficiently high superheating of the steam, the induction coil can be switched on in order to reheat the molten metal.

Charging of the induction furnace (heating up the molten metal) and discharging of the storage device (superheating the steam mass flow) can be performed simultaneously and in parallel. Decisive is the strategy of the power supply for the induction coil. With simultaneous charging and discharging, the power requirement is to be considered station supply and reduces net power generation that can be fed into the grid and is remunerated. With temporal staggering between charging and discharging, reasonably priced excess electricity for heating can be purchased from the grid.

The thermal storage composite system according to the invention further enables a solar thermal power station to handle base loads without employing fossil fuels in that, during the day, high-temperature thermal storage devices 15 and 1 are charged with solar energy and, at night, the power station is operated with the energy stored in the thermal storage devices 15 and 1.

The high-temperature thermal storage composite system according to the invention consists exclusively of components which are constructed in a simple manner, as well as of non-critical construction materials and storage media. Operation can be also realised in a very cost-effective and reliable manner.

While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles. 

What is claimed is:
 1. A high-temperature thermal storage device comprising: a crucible; an induction heating device; a tubular duct in which a heat carrier flows that is liquid and/or gaseous; an electrically conductive storage medium that is contained in the crucible and that fills the crucible at least partially; wherein the induction heating device, when electric energy is applied, heats the storage medium; and wherein the tubular duct passes at least partially through the storage medium.
 2. The high-temperature thermal storage device according to claim 1, wherein the storage medium is a molten metal.
 3. The high-temperature thermal storage device according to claim 1, wherein the storage medium has a melting temperature exceeding 500° C.
 4. The high-temperature thermal storage device according to claim 1, wherein the storage medium (9) is liquid at a temperature of 700° C.
 5. The high-temperature thermal storage device according to claim 1, wherein the high-temperature thermal storage device is charged by the induction heating device and discharged via the heat carrier passing through the tubular duct.
 6. The high-temperature thermal storage device according to claim 1, including thermal insulation.
 7. The high-temperature thermal storage device according to claim 1, comprising a heat exchanger integrated in the tubular duct, wherein the heat exchanger transfers heat from the storage medium to the heat carrier flowing in the tubular duct (7).
 8. The high-temperature thermal storage device according to claim 1 in the form of a modified induction melting furnace.
 9. A thermal storage composite system comprising: a first thermal storage device; a second thermal storage device; wherein the first thermal storage device is charged and/or discharged via a heat carrier that is liquid or gaseous; and wherein at least a partial flow of the heat carrier, after having passed through the first thermal storage device, passes through the second thermal storage device to be heated up further.
 10. The thermal storage composite system according to claim 9, wherein the second thermal storage device is a high-temperature thermal storage device comprising a crucible; an induction heating device; a tubular duct in which a heat carrier flows that is liquid and/or gaseous; an electrically conductive storage medium that is contained in the crucible and that fills the crucible at least partially; wherein the induction heating device, when electric energy is applied, heats the storage medium and wherein the tubular duct passes at least partially through the storage medium.
 11. The thermal storage composite system according to claim 9, wherein the partial flow of the heat carrier is fed to the second thermal storage device via controllable valves and is superheated to a superheated partial flow.
 12. The thermal storage composite system according to claim 12, further comprising a mixing station, wherein the superheated partial flow is mixed with the residual flow of the heat carrier in the mixing station.
 13. The thermal storage composite system according to claim 9, wherein the first thermal storage device has a lower operating temperature than the second thermal storage device.
 14. A process for operating a thermal storage composite system according to claim 9, comprising the steps of: Charging and/or discharged the first thermal storage device by a heat carrier that is liquid or gaseous; Charging the second thermal storage device by electric energy; Discharging the second thermal storage device by a heat carrier that is liquid or gaseous.
 15. The process according to claim 14, wherein the electric energy that charges the second thermal storage device is renewably generated electric energy and/or electric energy available on a spot market.
 16. The process according to claim 14, comprising increasing or lowering an output of the induction heating device to make available demand energy to a power grid.
 17. The process according to claim 14, comprising providing several of said second thermal storage device and connecting said several thermal storage devices in series and/or in parallel.
 18. The process according to claim 17, wherein said several thermal storage devices each have different operating temperatures.
 19. The process according to claim 17, wherein said several second thermal storage devices contain a storage medium that is a molten metal and wherein eh molten metals of said second thermal storage devices are different. 